The invention relates to a gate drive circuit for, and in combination with, an insulated gate power transistor.
In the case of power transistors, for example MOS transistors or IGBTs, which have a large areal extent, it is important to explicitly test the quality of the gate oxide in the product. To this end, there are corresponding gate stress tests in the front end and back end processes of semiconductor fabrication in order to safeguard the quality of this oxide. It is more difficult to carry out tests in integrated circuits which comprise a power transistor, for example a power MOS transistor. In order to carry out the quality test on the gate oxide, in the case of monolithic chips, a corresponding contact pad is routed to the outside in order to carry out the gate stress test in the front end process. A test is no longer required in the back end process since the gate oxide can no longer be damaged during installation since it is no longer possible to directly access the gate oxide electrically.
If, however, the driving logic unit and the power transistor are installed in a common housing in chips (dies) which are separate from one another, the situation is different. In the front end process, the separate chips can be tested in a known manner, especially the power transistor and the quality of its gate oxide. In the back end process, these chips are processed separately and are electrically connected to one another and to the leadframe. During processing, however, the gate oxide of the power transistor may now be damaged, for example as a result of electrical discharges or in the case of poorly grounded machines which have direct electrical contact with the gate oxide during processing. The entire structure is then surrounded with molding compound and the gate oxide of the power transistor is then no longer directly accessible from the outside. If this gate oxide of the large power transistor has been damaged, then, during processing in the back end process, it is no longer possible to carry out a direct quality test on the gate oxide since the gate itself is no longer electrically accessible. Only indirect tests are then possible in this case. If the gate oxide has been damaged, the gate is no longer insulated with respect to the source electrode and drain electrode or collector electrode and emitter electrode but rather there is an electrical connection across the oxide.
In principle, a plurality of test modes are available in the back end process, said test modes being able to detect the damage to the oxide. These include the measurement of the charging and discharging times of the power transistor which are affected by the damage to the oxide. However, these tests have the disadvantage that they can detect only coarse gate oxide errors with a large fault current since they are not sensitive enough to detect small initial damage to the gate oxide with a small fault current. During long-term operation, such small damage to the gate oxide may result in uncontrollable failure of the power transistor.
It would therefore be advantageous to specify a gate drive circuit of the generic type which makes it possible to detect such initial damage to the gate oxide in good time.